In recent years, sidewall interconnects have been studied to form fine interconnects exceeding the limit of lithography. For example, the sidewall interconnects are formed on both sidewalls of a core material which is formed of insulator. In this case, it is considered to reduce the capacitance between the sidewall interconnects by removing the core material after the sidewall interconnects are formed and forming an insulator with a low permittivity or an air gap between the sidewall interconnects, However, when the core material is removed, there is a high probability that the sidewall interconnects fall down. Therefore, a technique which can achieve reduction in capacitance between the interconnects while preventing falling of the interconnects is needed.